1. Field of the Invention
The invention relates generally to the devices for controlling coaxial output elements which are capable of pivotal displacement with respect to each other such as the control surfaces of an airplane with respect to the wings, and more particularly rotary ball actuators.
2. Description of the Prior Art
It is in fact known that the increasing speeds of airplanes (supersonic fighter aircraft) and stealth (insensitivity to radar detection) call for continuous surfaces or in other words surfaces without any projecting portions in order to cause the least possible disturbance to the aerodynamic flow.
In order to achieve this result, three families of solutions are in use: the epicyclic-gear rotary actuators; the vane-type hydraulic rotary actuators; and the ball-type rotary actuators.
The first family poses problems of rigidity, chiefly if it is desired to have a high torque in a thickness having the lowest possible value.
The second family poses problems of leakage (therefore of efficiency).
The third family, namely of ball-type rotary actuators ("rotary ball actuators"), offers substantial advantages in regard to efficiency, rigidity and service life.
However, taking account in particular of high loads and vibrations, correct operation of a rotary ball actuator depends on the need to ensure that the balls do not accumulate along their tracks.
With this object in view, one design approach has led to the construction of recirculating-ball actuators as described in U.S. Pat. No. 4,134,329 granted to the present Applicant (the teachings of which are incorporated in the present Application), or in U.S. Pat. No. 4,738,415. An actuator of this type usually proves satisfactory. Reference can also be made to the article entitled "Rotary Servohinge Actuator" by George Miller Jr. and Mark Williams, published in SAE Technical Paper Series in 1989 (ISSN 0148-7191) and illustrating a more elaborate version of a recirculatingball rotary actuator.
However, the recirculating-ball circuit is attended by two relatively undesirable consequences: an increase in the number of balls and an increase in width of the actuator, thus resulting in increased weight.
In patent No. EP-A-151 788, it was proposed to dispense with the recirculating-ball circuit and to replace it by a design consisting in particular of an intermediate sleeve which serves to mount two helical ball tracks in superposed relation. This design provides only a partial solution to the weight problems mentioned above and, in addition, involves technical difficulties from the point of view of construction.
The object of the invention is to obtain a rotary actuator with non-recycled balls, which is not subject to the disadvantages mentioned above and permits a significant reduction of overall size and weight.